Placental insufficiency is the main reason for fetal mortality, stillbirth, and early neonatal mortality. This is particularly a problem in farming, where a high production of viable offspring is desired. Healthy European pigs, for example, produce an average of about 12 piglets per litter, while the average loss is approximately 1 piglet per litter due to late fetal mortality, 1 piglet per litter due to stillbirth and 1 piglet per litter due to early neonatal mortality.
The term “fertility” as used in the present application and claims, is to be understood in a broad sense, viz, this term not only covers a decreased embryonic and fetal mortality, but also a decrease in perinatal and early neonatal losses of offspring, as well as a decrease in within-litter variation in development at birth. The improved fertility is reflected e.g. in an increased production of offspring for farming animals, such as pigs, or in an increased chance for viable offspring in case of animals, such as bovine or equine species or humans, which generally give birth to only a single individual per parturition.
Perinatal mortality is defined as the mortality within a few days around birth, viz, all mortality within a day or two before parturition, mortality during and immediately after parturition and all early-neonatal mortality.
In a commercial pig-breeding unit, the overall profitability rises with an increasing number of piglets weaned per sow per year. The latter depends on age at first farrowing, breeding life expectancy, the prolificacy of the sows in the herd (i.e. litter size), number of farrowings per sow per year and perinatal and pre-weaning mortality rate. Considering the fact that for each weaned piglet more than two oocytes were initially fertilized, it is evident that piglet mortality during pregnancy, parturition and lactation imposes a substantial loss to the farmer.
In order to prevent fetal mortality and thus improve the fertility of animals, it has inter alia been suggested to increase uterine capacity by genetic selection, as described for pigs by Leymaster et al. (Proceedings of the 3rd World Congress on Genetics Applied to Livestock Production, XI, (1986) 209-214) or by selecting for increased placental efficiency (Ford, Journal of Reproduction and Fertility, Supplement 52 (1997) 165-176; Wilson et al. Journal of Animal Science, 77 (1999) 1654-1658). Although it is possible to select for uterine capacity, this route requires considerable time and effort, and is particularly difficult because uterine capacity is not a simple trait to measure.